Of particular interest are discrete systems where, rather than modulating a carrier with a continuously variable information signal, successive time periods (“symbols”) of the carrier each serve to transmit one piece of information; that, is, the modulated information does not vary during the course of a symbol.
Of the most practical interest is the situation where the information to be sent is in digital form, so that each symbol serves to transport a number of bits, but this is not in principle necessary and sampled analogue signal could be sent i.e. the information signal is quantised in time but may or may not be quantised in amplitude.
Quadrature modulation may if desired be used, where both the phase and amplitude of the carrier are varied, or (which amounts to the same thing) two carriers at the same frequency but in phase quadrature may each be modulated independently. A “multicarrier symbol” may thus consist of a time period during which are transmitted (say) 256 carriers at different frequencies plus 256 carriers at the same set of frequencies but in phase quadrature. For digital transmission, up to 512 groups of bits may be modulated onto these carriers. Normally the carriers are harmonically related, being integer multiples of the symbol rate (though in systems using a “cyclic prefix” the symbol rate is slightly lower than this statement implies). This form of modulation is particularly attractive for use on poor quality transmission paths, since the number of bits allocated to each carrier can be tailored to the characteristics of the path, and indeed carriers may be omitted in parts of the frequency spectrum in which quality is especially poor.
The number of bits sent on each sub-channel may if desired be varied depending on the signal and noise levels in each sub-channel. This can be a particular advantage for transmission paths which suffer crosstalk or radio frequency interference, since the system can adapt automatically to avoid regions of frequency spectrum that are unsuitable for data transmission. The number of bits sent on each sub-channel may if desired be varied adaptively depending on the signal and noise levels in each sub-channel as observed from time to time. This can be a particular advantage for transmission paths which vary significantly over the course of a communication.
Multicarrier modulation has been standardised for use on copper pair links in a form known as discrete multitone (DMT) modulation. This is described in an ANSI standard (T1.413-1998) for asymmetrical digital subscriber loop technology and also a European standard [DTR/TM-03050] and an international standard [ITU G.adsl].
A modulator for multicarrier systems may be constructed with a bank of oscillators at the respective frequencies, each followed by a modulator, whilst a receiver might consist of a bank of synchronous demodulators each driven by an oscillator synchronised to the corresponding oscillator at the transmitting end. In practice, however, a more popular approach is to regard the data values to be transmitted for a given symbol as Fourier coefficients and to generate the modulated signal by means of an inverse Fourier transform. Similarly the demodulator would apply a Fourier transform to the received signal in order to recover the transmitted carrier phase and amplitude (or in-phase and quadrature components) which can then be decoded using standard quadrature amplitude modulation (QAM) techniques. Such a demodulator, as envisaged by the above-mentioned ANSI standard, is shown in FIG. 1. The received signal is filtered by a filter 1, and converted into digital form in an analogue- to digital converter 2. The digitised samples are entered into a buffer 3, synchronisation being provided by a control unit 4 so that, for each symbol, a block of 512 samples is assembled in the buffer. These are then supplied to a discrete Fourier transform unit 5 which processes the samples to recover complex values zj(j=0 . . . 254) representing the transmitted carrier (plus of course, noise), output as in-phase and quadrature components lj, Qj(that is, zj=lj+iQj). These are scaled at 6, each zj being multiplied by a complex number to compensate for delay and attenuation suffered by the relevant carrier, and then fed to a QAM decoder 7 (usually employing some form of convolutional code and a soft-decision decoder), whereby the desired data values are recovered.
One of the functions of the control unit 4, in addition to synchronisation, is to engage, at start-up, in a training sequence, that is, a dialogue with the transmitting modulator in which it obtains the information it needs about the transmitted signal, for example, which sub-channels are actually in use, how many bits are carried by each sub-channel, and what QAM constellations are being used by the modulator. In some systems, these parameters may be changed dynamically by further negotiation between the two ends during actual transmission. It is noted that the timing output from the control unit 4 serves for synchronisation of the various parts, whilst the control output indicates which sub-channels, and which constellations, are currently in use.
The invention is defined in the claims.